New research into homogeneous catalysts looks to replace precious metal based …

Homogeneous catalysis, in which the catalyst is mixed directly in with the reaction components, sees widespread use in industrial settings. The catalysts themselves are often complex organometallic compounds that contain a precious metal atom/ion—platinum, rhodium, palladium, rhenium—at their molecular center.

From an engineering standpoint, a reactor for a homogeneously catalyzed reaction can often be described as a catalyst recovery system first, reactor second. The high cost of these precious metals means that recovery and reuse of the catalyst is essential to making the reactions economic.

A report published in last week's edition of Science discusses the work of a team of chemists who are looking at ways of obviating the need for the precious metals, replacing them with their more ordinary relatives. The paper focuses on chemistry that is important to the silicone industry.

The authors point out that in 2007, the worldwide silicone industry used 5.6 metric tons (12,345 lbs) of platinum in its various reactions, and a lot of it was lost in the product stream and could not be recovered. As of publication time, platinum is trading at $1,623 per troy ounce—that means the silicone industry, assuming their usage levels are the same as 2007, will spend just shy of $300 million on platinum alone this year. The authors point out that platinum is becoming rarer and rarer and finds itself in other high-tech uses such as fuel cells, so its price will continue to rise and see wild fluctuations.

The focus of this work is hydrosilylation reactions, which link a tertiary silane (something of the form R1R2R3-Si-H, where each R represents some organic functional group) to an existing carbon-carbon double bond. Normally, these reactions involve a platinum catalyst, but the researchers managed to substitute an iron-based one. They found that "in many examples, the new base-metal (iron) catalysts offer advantages in both activity and selectivity over currently employed precious compounds." Basically, their new catalyst worked as well or better than what we're currently using in each of the reactions they looked at.

As a base test case, the team studied the formation of 3-octyl-1,1,1,3,5,5,5-heptamethyltrisiloxane (for those who despised organic chemistry like I did, this is basically a silicone compound terminated by seven methyl groups hanging off the end of an octane chain). It's a compound commercially used in agricultural and cosmetic products. The reagents used to synthesize this compound were (Me3SiO)2MeSiH, termed MD'M, and 1-octene. In the presence of the iron catalyst and fairly mild conditions (barely above room temperature), 2000 ppm of the catalyst would yield over 98 percent conversion to the desired product.

From a chemistry standpoint this is great news—near complete conversion is considered a positive, and there was no evidence of any other side reaction products. That means no additional purification before adding the results to commercial products.

The researchers also compared the results to those obtained using the more traditional Karstedt's catalyst (PDF). That requires a reactor temperature of 72 oC and a 30 ppm precious metal catalyst. It only went to around 80 percent conversion and, even worse—from a chemical engineering standpoint—it produced a sizable quantity of byproducts from alkene hydrogenation, isomerization, and dehydrogenation. (The authors note, however, that researchers have discovered ways to suppress these side reactions using a different type of precious metal catalyst.)

The authors looked at a second class of reactions: the hydrosilylation (same idea as before) of hydrido- and vinyl-functionalized silicon fluids. These reactions result in a cross-linked silicon polymer (think something akin to vulcanized rubber) that finds uses in the coatings industry. Here, the reaction product is highly viscous and has a morphology that makes recovering much, if any, of the catalyst a major technical challenge.

Here again, the researchers found that the addition of only 500ppm of one of the iron catalysts in a silicone fluid solution resulted in a nicely crosslinked polymer within two hours. As an added bonus, this process does not require the use of toluene, which is needed in the existing reaction to to get the fluid and catalyst to play nicely with one another. Analysis of the final product showed that it was structurally identical to those prepared commercially using platinum catalysts.

Based on the Science article, it seems like this class of bis(imino)pyridine iron dinitrogen complexes provides a promising route for industrially relevant reactions. The simple fact that it replaces a high cost material with a readily available and cheap metal is a boost right away. We still need to know how readily (affordably) the iron catalysts can be made and how easily they can be separated back out from the product stream. Even if it is cheaper then platinum, no one wants to see their catalyst only get used once and then be lost forever.

34 Reader Comments

Does this maybe imply that the catalyst is more due to the shape of the chemical structure and not the specific components?

I know in biological systems the shape is very important, some times more so than the specific chemical components. This is how many poison, drugs, or hormone mimics seem to work. That the chemical shape is close enough to the real item that it triggers whatever the real item should but then is either not controled by the real item, or is to much...

i am looking forward to reading new advances in using/creating catalyists... These are the changes that will allow for technological sustainability as the population continues to rise. It would be great to replace all these precuious metal cataylysts with the universe's unlimited quantity of iron.

Does this maybe imply that the catalyst is more due to the shape of the chemical structure and not the specific components?

You're right, the shape of the molecule is very important, but generally only insofar as it affects reactivity of the metal. Most (all?) human-made catalysts do covalent catalysis, that is they form bonds with the substrate (the molecule that they transform). This is a bit different than the catalysis performed by (some) enzymes, where the shape of the active site can have a huge impact on the reactivity (often in conjunction with covalent catalysis). Unfortunately we can't make any arbitrary shapes like an enzyme can, and small molecules cant enclose reactants like enzymes, so we're limited in what we can do.

hell yes, can we have more chemistry articles?most articles on this site are in the physics/biology realm.

i'm an organic chemist and my father is a polymer chem E.thanks for the article review matt, keep them coming !

it is interesting to see them switch from pt to fe, where there is obviously an electron difference, but also a difference in common oxidation states. also, pt is typically square planar, but these new molecules look square pyramidal?

Does this maybe imply that the catalyst is more due to the shape of the chemical structure and not the specific components?

I know in biological systems the shape is very important, some times more so than the specific chemical components. This is how many poison, drugs, or hormone mimics seem to work. That the chemical shape is close enough to the real item that it triggers whatever the real item should but then is either not controled by the real item, or is to much...

The answer is highly specific to the catalyst. The rate of reactions in solution is usually governed by a single energetic barrier at the transition state (Arrhenius/Eyring behavior) that accounts for the slow rate of the reaction. The reaction rate basically measures the probability of the reactants experiencing a thermal fluctuation that "pushes them" over this barrier.

Enzymes often work by providing a local environment that changes the energetics of the existing pathway, either by destabilizing the reactants, or stabilizing the transition state of a reaction pathway. The net effect is a decrease in the energy difference between the reactants and the transition state, which makes these fluctuations more probable. The "shape" of the active site is quite important here, as it is largely what defines the local environment.

In contrast, chemical catalysts rarely control the local environment in this way. Instead, they often provide an entirely different pathway, with different underlying energetics. As a previous poster mentioned, chemical catalysts typically bond with one, or both, of the reactants. Traditional hydrogenation provides a decent example of this:http://chemwiki.ucdavis.edu/Organic_Che ... rogenation

Because chemical reactions are highly dependent on the electronic configuration of the molecular orbitals, the elements involved (components) do matter, but so does the geometry of the catalyst.

As of publication time, platinum is trading at $1,623 per troy ounce—that means the silicone industry, assuming their usage levels are the same as 2007, will spend just shy of $300 million on platinum alone this year.

this is great news, and hopefully this can bring down the price of products from the silicon industry and make our gadgets cheaper in the short term (not 10 years out).

perhaps precious metals will soon get to a price point high enough where we will start mining asteroids for them. that would be pretty awesome. if a robotic miner could land, mine a couple tons, and fly back, it could be a quite viable and profitable venture. (ps, i'm not ferengi)

" Even if it is cheaper then platinum, no one wants to see their catalyst only get used once and then be lost forever."

Why? If its cheaper overall to use the catalyst once and discard it as compared to recovery and reuse, then why would anyone put any effort into recovery (assuming no enviromental damage or damage to the end product)?

" Even if it is cheaper then platinum, no one wants to see their catalyst only get used once and then be lost forever."

Why? If its cheaper overall to use the catalyst once and discard it as compared to recovery and reuse, then why would anyone put any effort into recovery (assuming no enviromental damage or damage to the end product)?

Cheaper is really a relative term here. Even if the catalyst doesn't contain precious metals, it is a highly complicated organometallic molecule that is not trivial (and hence, not cheap) to make.

More specifically, rust is iron oxide, which can be produced under oxidizing conditions, and will require oxygen to come from somewhere (usually it's atmospheric oxygen). The reaction this is catalyzing is a reduction reaction, which will prevent rust from forming. If anything, the more likely problem would be the organometallic iron getting over-reduced and turning into Fe metal. This would no longer have the required organic molecule--the "ligand"--attached, and thus not catalyze the reaction desired.

If a cheaper, more common and easier to work with material provides notably better performance in an application, why did we ever use the expensive, rare and awkward material?

Seconded...why did we even use platinum to begin with? I'm a chemistry n00b, so a little blurb about that would have been appreciated.

I find it hard to believe that companies spending millions on the research for these processes would have overlooked something that's turning out to be blatantly cheaper and better...though it's not entirely impossible they got stuck on this rare metal stuff and missed it.

EDIT: I did notice hobgoblin's reply, but just because it's more complex doesn't mean it couldn't have been investigated at the time. This article (the Ars one...at least how I read it) describes something nobody really looked into at all. From a chemist's perspective I can see them ignoring iron and sticking with platinum because it's easier. But, from an accountant's standpoint, I'm surprised they were allowed to even look at platinum, even if iron had some complexity attached.

Are there more factors at play in this case? (ie, the iron thing doesn't scale well or didn't back when these processes were first made...etc.)

The reason it wasn't used before? It requires specifically tailored compounds to make the catalysts used. The platinum can be used as platinum metal, whereas this iron requires the organic ligand shown in the picture to work like it does. There is a HUGE number of compounds available to test, and we're finally getting some good results, like in this story.

Cosmetic products like those in http://www.nuaveworld.com/ often include the 3-octyl-1,1,1,3,5,5,5-heptamethyltrisiloxane compound. I've seen it in many of such products, and it is a very poplar ingredient indeed.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.